This invention relates to the field of seismic signal processing and more specifically to the processing issues involved in orientation of multi-component detectors (a.k.a. geophones).
In the area of seismic signal processing, there has been a desire to analyze both shear (S-wave) and pressure (P-wave) data. For example, it has been discovered that S-waves do not respond to some hydrocarbon structures in the manner that P-waves respond. P-waves may reflect strongly at structures that are both water-containing, and therefore not economic for drilling, and hydrocarbon-containing. S-waves, on the other hand, will reflect strongly at many of the same water-structures as P-waves, but they do not reflect strongly at many of the hydrocarbon-containing structures. Therefore, comparison of S-wave displays and P-wave displays of a given structure helps in making decisions regarding which structures should be drilled.
Further, some S-waves reflect differently at certain structures than other structures, which is another indication of rock properties in which an interpreter may be interested. Therefore, comparison of S-wave displays of different types and at different orientation angles is desired by interpreters. There are many such differences, which are known to those of skill in the art. See, e.g., J. P. DiSiena, J. E. Gaiser, and D. Corrigan, 1981, Three-Component Vertical Seismic Profiles: orientation of Horizontal Components for Shear Wave Analysis. 51.sup.st Annual International Meeting, Society of Exploration Geophysicits, Expanded Abstracts, 1991-2011; R. H. Tatham and M. D. McCormack, 1991 Mulitcomponent Seismology in Petroleum Exploration. Society of Exploration Geophysicists; J. F. Arestad, R. Windels, and T. L. Davis, 1996, Azimuthal Analysis of 3-D Shear Wave Data, Joffre Field Alberta, Canada. 66.sup.th Annual International Meeting, Society of Exploration Geophysicists, Expanded Abstracts, 1563-1566; J. E. Gaiser, P. J. Fowler, and A. R. Jackson, 1997, Challenges for 3-D Converted-Wave Processing. 67.sup.th Annual International Meeting, Society of Exploration Geophysicists, Expanded Abstracts, 1199-1202; R. R. VanDok, J. E. Gaiser, A. R. Jackson, and H. B. Lynn, 1997, 3-D Converted Wave Processing: Wind River Basin Case History. 67.sup.th Annual International Meeting, Society of Exploration Geophysicists, Expanded Abstracts, 1206-1209; and references cited therein; all of which are incorporated herein by reference.
One of the ways of detecting the S-waves is with the use of so-called "multi-component" detectors. These geophones have sensors oriented to receive seismic signals from two horizontal directions (the in-line and the cross-line directions) and one vertical direction. Theoretically, a signal moving along the inline axis in the positive direction will generate a positive response on the in-line geophone component. The cross-line component will not respond at all to such a signal, nor will the vertical component. Likewise, a signal moving in the cross-line axis will generate a response at the cross-line geophone, but not on the other two components. Most signals, of course, travel at an angle to the cross-line and in-line directions, and they generate cross-line and in-line components whose amplitude is dependent upon the angle of incidence of the signal to the component. This is especially true in 3D (three dimensional) surveys where the source for some of the shots is not in-line with the receiving cable.
For example, referring to FIG. 1, a simplified example of a 3D multi-component survey is seen, in which there are several sources S1-S8 located around a single receiver R. As shown, the horizontal components H1 and H2 of the receiver R are oriented differently for each of the sources S1-S8. FIG. 1 is illustrative only. An actual 3D geometry is, of course, more complex. FIG. 2 shows example synthetic data recorded on the two horizontal components of the geophones of FIG. 1. Each source generates an exponentially tapered sine wave whose initial amplitude is one, and the direction of the particle motion is in the geophone-source plane. From these 16 traces, it is seen that the amplitudes and polarity vary from trace to trace. Therefore, the amplitudes and polarity must be adjusted before any further processing is applied. To make the proper adjustments, there must be knowledge of the orientation of the horizontal components of the geophones; and, there is a need for a simple and effective method for determining that orientation.
Interpreters normally require displays of data in at least two directions (the "radial" and, "transverse" directions). The data must be corrected for variations in the orientation of the horizontal components, since the amplitude of the data recorded from a particular component depends upon the angle of incidence of the signal to the component's direction of reception.
In positioning of the multi-component receiver cables in the survey, ideally, the in-line orientation would all be known, as would the cross-line orientation. This is somewhat practical in land surveys where care in the layout of the cable is taken. It is also somewhat possible in marine "dragged array" surveys, where the cable is dragged in a particular direction after deployment, giving the multi-component geophones substantially the same orientation. However, in ocean-bottom surveys ("OBS") in which the receiver cable is not dragged, and in vertical seismic surveys where the geophones are placed in a well bore, the orientation is more random, or perhaps even reversed, due to the twisting and coiling of the cable during deployment. Further, even where the orientation is generally known, such as in land and dragged array surveys, the orientation is not perfect, and as much as a ten percent difference can exist between any two receivers. There is a need, therefore, for determining the orientation of multi-component receivers. See, e.g., DiSiena, et al., Three-Component Vertical Seismic Profiles: Orientation of Horizontal Components for Shear Wave Analysis, 1981 Society of Exploration Geophysics Annual Meeting Papers (also incorporated herein by reference).
Earlier attempts at orientation determination include the so-called "hodogram" method described in the 1981 DiSiena paper cited above, in which the amplitude of the in-line component is plotted against the amplitude of the cross-line component. A line is then best-fit to the resulting set of points, which gives the direction of the components. However, this process has been found to be time consuming, and it fails to give the polarity of the waveform. As discussed above, where the orientation of the components is unknown, as in the non-dragged OBS cable or the VSP arrays, this is a serious drawback.
Another method of determining the angle is also described in the 1981 DiSiena paper, in which a mathematical rotation of the components is performed until the energy seen in one of the components is maximized. Again, such a process is time-consuming, expensive, and cannot give the polarity information needed.
Accordingly, there is a need for a simple, inexpensive, and fast method for determining the orientation of horizontal components of receivers.